How to align imported mesh to grid?
You can hit CTRL+ALT+Q to toggle quad view mode, showing your model along all major axes.

From there, you can rotate with r for manual corrections. For finer control, hold Shift while you rotate.

Edit: An alternative method to ensure a symmetric mesh when you're done:


1 to view from front
Tab to enter edit mode
z to display wireframe
b+box select the whole left side of your mesh (you may need to Ctrl+r add an edge loop down the mid-line first)
x delete selected vertices
Add and apply a mirror modifier along the x axis


Not sure how this will differ from what you described in your comment, but it's an option